1. Field of the Invention
The present invention relates to a process for trapping a catalyst metal strongly involved with the adhesion of a metallic deposit, or electroless plating, to a substrate in an electroless plating process. Specifically, the present invention relates to a process for trapping and fixing a catalyst metal as catalytic nuclei on the surface of a substrate through selective and strong chemisorption of the catalyst metal from a solution containing the catalyst metal by the action of chitosan or a chitosan derivative contained in a pretreatment agent.
2. Description of the Prior Art
Non-conductive plastics, ceramics, paper, glass, fibers, etc. can be plated by electroless plating. In order to initiate oxidation of a reducing agent in a plating solution, however, the surface of such a non-conductive substance as mentioned above must be subjected to a catalyzation treatment. Although a known classical catalyzation treatment method is a sensitizing-activating method wherein are employed a stannous chloride bath and a palladium chloride bath, a catalyst-accelerator method wherein are employed a stannous chloride-palladium chloride bath and a sulfuric acid (or hydrochloric acid) bath is now generally employed as a catalyzation treatment method. Further, another catalyzation treatment method has recently begun to be adopted, wherein a substrate is immersed in a solution of a palladium complex having a strong adsorbability and then washed with water, followed by precipitation of palladium metal with a reducing agent such as dimethylaminoborane.
The step of pretreatment useful for the step of catalyzation is an etching step required in order to secure a wettability (hydrophilicity) of the surface of a substrate for promoting the physical adsorption thereon of a catalyst metal. In such an etching step, a chromic acid etching solution is now used for plastics and the like in most cases. In the chemical etching step, the surface of a substrate is microscopically roughened to facilitate physical trapping thereby of a catalyst metal in the catalyzation step while securing an anchoring effect involved with the adhesion of the resulting metallic deposit, or plating, to the substrate. In this sense, the chemical etching step is a very important step (see FIGS. 1 and 2).
According to the sensitizing-activating method, which is a two-stage process, a substrate is first immersed in a solution of stannous chloride in the sensitizing step thereof to adsorb Sn.sup.2+ on the surface of the substrate, and then treated with a solution of palladium chloride in the activating step thereof to precipitate Pd nuclei according to a redox reaction represented by the following formula: EQU Sn.sup.2+ +Pd.sup.2+ .fwdarw.Sn.sup.4+ Pd
Chemicals to be used in sensitizing, i.e., sensitizers, have heretofore been studied since old days including those as disclosed in patents dating from around 1936 (U.S. Pat. No. 2,063,034 (Dec. 8, 1936). The kind of sensitizer is not so often varied depending on the kind of substrate and the kind of electroless plating. A variety of hydrochloric acid-acidified solutions of stannous chloride used solely as a principal ingredient are used in the sensitizing step. Proposed sensitizers other than stannous chloride include platinum chloride and titanium chloride, which may also be used in the form of a hydrochloric acid-acidified solution. On the other hand, a solution of palladium chloride (0.2 to 1 g/l, hydrochloric acid: 5 ml/l) is most widely used as the activating solution. Salts of precious metals such as Pt, Au and Ag other than Pd are also effective for an electroless copper plating solution.
The catalyst to be used in the catalyst-accelerator method is a mixed solution of stannous chloride and palladium chloride with hydrochloric acid, which is commercially available in the form of a concentrated solution, which is usually diluted with a large amount of a solution of hydrochloric acid to be ready for use thereof. The catalyst-accelerator method is carried out at a treatment temperature of 30.degree. to 40.degree. C. for an immersion time of 1 to 3 minutes. 5 to 10 vol. % sulfuric acid or hydrochloric acid is generally used as the accelerator, which may alternatively be a solution of sodium hydroxide or ammonia. Rantell et al. reported that the mixed solution of stannous chloride and palladium chloride with hydrochloric acid is not colloidal, but a solution of a complex salt having a composition: SnPd.sub.7 Cl.sub.16 and solubilized in the presence of surplus stannous chloride. Further, Rantell et al. drew the following inference as to the progress of a reaction in the accelerator step [A. Rantell, A. Holtzman; Plating, 61, 326 (1974)].
In the catalyst step, the Sn.sup.2+ -Pd.sup. 2+ complex salt is first adsorbed on the surface of a substrate, and the adsorbed complex salt is then hydrolyzed when the substrate is washed with water. Through the hydrolysis, tin is precipitated in the form of an Sn(OH)Cl precipitate, which is in a state of coexisting with tetravalent tin and the palladium salt. In the following accelerator step, the precipitated stannous salt is dissolved and then reacted with the palladium salt already relieved of a complex salt state to yield palladium metal according to the following redox reaction: EQU Sn.sup.2+ +Pd.sup.2+ .fwdarw.Sn.sup.4+ Pd
As a result, palladium metal and small amounts of bivalent and tetravalent tin salts remain on the surface of the substrate.
The reaction mechanisms involved in the sensitizing-activating method and the catalyst-accelerator method as the conventional catalyzation methods for electroless plating have been substantially elucidated as described hereinbefore. In any case, however, many reactions are involved until catalytic nuclei of a metal such as palladium are precipitated. Accordingly, the catalyst metal is lost little by little in the form of various reaction intermediates formed by the respective reactions every time when washing of a substrate with water, and the like are effected for every such reaction. The final residue of the catalyst metal is greatly affected by many factors such as the concentrations, pH values and temperatures of solutions used in respective steps, and the immersion periods of time for such solutions, as well as the conditions of degreasing and roughening of the surface of the substrate. Accordingly, when the final uptake of the catalyst metal is insufficient, the adhesion of the resulting metallic deposit to the substrate is always imperfect to cause a failure in plating.
The foregoing phenomena are attributed to mere "physical adsorption" of such catalyzation reaction intermediates and the metal catalyst trapped into recesses and micropores in the surface portion of the substrate microscopically roughened by chemical etching.